spvtest::Validate::CompileFailure: Don't leak the diagnostic (#2564)

diff --git a/test/val/val_fixtures.h b/test/val/val_fixtures.h
index 79ad725..5635c78 100644
--- a/test/val/val_fixtures.h
+++ b/test/val/val_fixtures.h
@@ -21,6 +21,7 @@
 #include <string>
 
 #include "source/val/validation_state.h"
+#include "spirv-tools/libspirv.h"
 #include "test/test_fixture.h"
 #include "test/unit_spirv.h"
 
@@ -113,6 +114,7 @@
             spvTextToBinary(ScopedContext(env).context, code.c_str(),
                             code.size(), &binary_, &diagnostic));
   std::string result(diagnostic->error);
+  spvDiagnosticDestroy(diagnostic);
   return result;
 }